Interface is a single sided BC (just like a wall). Therefore Fluent creates a second boundary condition to represent the other side.

I believe this isn't an issue for "interior" BC's in Gambit, but it's been a while since I used one of those. If you don't want the face to export, then don't assign a BC to it. If it's truly an interior face (volume mesh on both sides and fully connected) then it won't export unless you tell Gambit to export it as an interior BC.

What information are you trying to pass? The "interface" BC has a specific definition, and it is a way of passing fluid from one side to the other (used for non-conformal meshes, sliding interfaces, etc). This doesn't sound like what you have.

If this is a surface that flow can pass freely through, then use the "Interior" BC. The surface will exist for plotting and such, but will not effect the solution itself.

If this is a porous wall (i.e. you want to have a pressure drop as the flow passes from one concentric circle to the next), then use the porous-jump BC.

When you select one of those conditions, the "shadow" boundary condition automatically goes away.

If this is a solid wall, then you're going to have the "shadow" walls. You can still pass heat through. When you turn on the energy equation and then define the boundary condition for the wall you'll see that the shadow wall shows up in the "Shadow Face Zone" box. Then under the thermal tab, you can select coupled (this is the default setting) and you can assign material properties/thickness to the wall. Fluent knows to pass heat through this wall, but won't allow fluid through.

When you enable the energy equation, Fluent will automatically couple the wall and the shadow zone for heat transfer. You can double check this in the boundary conditions panel, but it should happen automatically. Are you trying to do something else?

thanks...i am trying to solve a problem that contains some concentric cylindrical of different materials...so the wal in between the material i need it to be act as interface...all the materials are solid....except one which is fluid that is the inner most cylinder.there is no heat generation.so will i use interior or inteface in the wal?

When you exported your mesh from Gambit, you must make sure you assign a different continuum to each of the different materials. Then use the wall BC. If you really want to use the interface BC, then in Gambit you should disconnect the faces/edges before exporting the mesh.

The only benefit to the interface is an unsteady problem where you're going to have the grids physically rotate relative to one another (sliding meshes). If the cylinders aren't moving, then stick with the wall BC.